1. Field of the Invention
The present invention relates to a heat sink and a projection display apparatus using the same, and more particularly, to a forced air-cooled heat sink.
2. Description of the Related Art
Electronic devices tend to generate a larger amount of heat because of the requirement for higher performance and reduction in size. Therefore, various methods have been proposed for efficiently cooling such electronic devices. For example, Japanese Patent Laid-open Publication No. 2004-193389 discloses a heat sink which employs a refrigerant. In order to improve the characteristic of heat transfer and to cool each electronic device at a uniform temperature, heat transfer coefficient of a heat sink with regard to the refrigerant which flows inside differs at locations along a flow path, depending on the arrangement of electronic devices that are to be cooled. Japanese Patent Laid-open Publication No. 115156/95 discloses a heat sink, which employs a refrigerant, for cooling integrated circuits. In order to improve cooling efficiency and to reduce pressure loss, a protrusion, which is arranged beneath a refrigerant nozzle, is formed on the bottom of a heat sink, and corner areas are cut away to eliminate dead water zones. In a cooling apparatus disclosed in Japanese Patent Laid-open Publication No. 159070/95, through-holes which extend parallel to each other are provided inside the cooling apparatus, and through-holes adjacent to each other are connected together by U-shaped connection pipes to form a continuous refrigerant flow path.
In such a liquid-cooled heat sink employing a refrigerant, a closed loop is generally formed to allow the refrigerant to circulate from a pump to a heat receiving jacket (heat sink), a radiator, and back to the pump. A reservoir tank may also be used, as needed, in order to accommodate leaks and evaporation of cooling water. In a liquid-cooled heat sink, heat is radiated by a radiator. Since the radiator is an air-cooled heat sink having fins on the outer surface thereof, cooling air, similar to conventional heat sinks, needs to be supplied from surroundings. In other words, the refrigerant only transfers heat from a device that is to be cooled to the radiator, and it does not actually cool the devices. Consequently, an air-cooled heat sink system has been increasingly used to cool an electronic device, because it directly cools the heat sink for an electronic device by means of a blower, and, as a result, it facilitates the simplification of a cooling structure.
On the other hand, noise reduction has been required for home electric appliances, and technologies for noise reduction have been developed. This tendency is not limited to home electric appliances, and is being extended to general electronic devices. In particular, in the field of personal computer-related apparatuses, noise reduction is highly required for a peripheral device of a computer system such as a projection display apparatus, irrespective of the size of the apparatus, as well as for components of a computer system such as a magnetic disk drive, a CPU (Central Processing Unit) cooler, and a power supply cooler.
Since a projection display apparatus is equipped with components that generate particularly large amounts of heat, the components that generate heat need to be cooled in order to ensure performance and reliability. The cooling system is roughly classified into two types, i.e., air cooling system and liquid cooling (water cooling) system. The air cooling system which is used to cool electronic devices is further classified into natural air cooling system and forced air cooling system, and the latter is usually used because the former needs a wide heat transfer area due to low cooling efficiency. In the forced air cooling, a blower, which may be of various kinds, forces cooling air to flow against an object that is to be cooled, or exhausts cooling air that is heated in an apparatus. A heat sink may be provided to cool an optical component which is heated to a particularly high temperature. In order to enhance the effect of cooling an electronic device, it is necessary to employ a larger cooling blower or to increase the rotational speed of a blower. However, this may result in an increase in the noise of the cooling blower, and the increase in noise is still larger for an electronic device that is equipped with components that generate large amounts of heat. Furthermore, the increase in noise tends to be larger for small portable devices, because the rotational speed needs to be increased in order to limit the size of a blower.
In a conventional heat sink used in a projection display apparatus, an object is cooled by arranging a heat sink in an air flow, and dissipating heat, which is transferred from the object to the heat sink, by the air flow. In order to cool an object in this way, a space is required that allows cooling air to flow around the heat sink. Cooling air is heated to a high temperature, and causes the temperature to rise in the apparatus. Since heat dissipating efficiency of a heat sink is affected by the ambient temperature, heated air needs to be purged to the outside of the apparatus. Further, a large surface area is required to improve the heat dissipating efficiency of a heat sink, leading to an increased height of the fins. There is a need for a reduction in the size of an apparatus, as well as for the compatibility between cooling performance and noise reduction (lower noise) in a conventional projection display apparatus.
However, the aforementioned conventional air cooling technology that uses a heat sink has the following disadvantages. First, in a small projection display apparatus, it is necessary to operate a blower that is mountable in an apparatus at a high rotational speed in order to ensure a flow rate that is necessary to cool a projection display apparatus having a component that generates large amounts of heat. The increase in rotational speed is required in order to increase the flow rate and air pressure because a normal rotation speed is insufficient for a small blower to ensure a sufficient flow rate. However, the increase in rotational speed may result in larger noise. Further, rotation of a blower at a higher rotational speed tends to generate noise that is offensive to the ear. Therefore, even if noise level is kept low, the level of the noise may still seem to be higher because noise is offensive to the ear.
Secondly, a smaller projection display apparatus has a limited flexibility for the arrangement of components in the apparatus, as compared with a larger apparatus, leading to the difficulty of using natural air cooling that introduces cooling air from the outside of the apparatus.
Thirdly, since components occupy a higher ratio of internal space in case of a smaller projection display apparatus, as compared with a larger apparatus, it is more difficult to exhaust heat to the outside of the apparatus due to increased ventilation resistance inside the apparatus. The apparatus has to be cooled under the condition in which there is insufficient air flow inside the apparatus.